The Triton-3 platform is a novel tidal energy harvester capable of producing 3MW from tidal flow. The platform is a floating structure moored to the seabed by a single-point fully articulated anchorage, and carries three power trains and a number of marine auxiliaries. The driver for the design as developed by TidalStream Ltd is to reduce the cost of energy production in order to compete with the current cost of offshore wind. Independently audited cost modelling shows that tidal stream energy can become competitive with offshore wind, achieving a generating cost as low as 10p/kWh at the best sites. This generating cost is estimated to be less than half that which could be achieved at a similar site from a single seabed-located turbine.
The driving aspects for the competitive cost are maximising the capacity per mooring point, reducing installation costs by float-out solutions and by providing easy access to the tidal equipment. Access is achieved by allowing the platform to come to the surface by means of de-ballasting. By doing so, there is no need for large workboats and/or diver activities to perform regular inspection and maintenance on the tidal equipment, reducing the cost significantly.
The technical aspects that arise when developing the tidal turbine platform for a typical offshore location are investigated by Houlder Ltd and discussed in this paper. A number of technical challenges have been addressed where the rotational stability in both roll and pitch are of interest. The roll of the platform is heavily affected by the performance of the turbines; sudden increase or reduction in thrust will induce significant rolling moments that must not impair the integrity of the platform. Pitching of the platform allows it to reach the surface when de-ballasted for maintenance and inspection. During normal operations, the platform remains aligned with the current and in doing so maximises the performance of the turbines.
The paper illustrates how these aspects have been achieved by means of passive solutions. By means of positioning and shaping the main body of the platform, a working configuration has been developed where the rotations of the platform remain within a limited window maximising the potential power production. The concept has been tested by TidalStream during a large-scale model testing campaign where the unit was subject to different current speeds and different turbine configurations and fault cases. This publication compares the results of the large scale model testing with numerical models developed in OrcaFlex and shows the effectiveness of the passive solutions.